1. Field of the Invention
The present invention relates to a white light source such as an amplified spontaneous emission (ASE) light source. More specifically, the present invention relates to a white light source for a system for evaluating and measuring common optical parts, a white light source for a system to carry out evaluations or measurements for optical parts of an optical-fiber communication system using optical fibers, and a spectrum slice signal light source and a CDM (Code Division Multiplexing) signal light source.
2. Description of the Related Art
In recent years, efforts have been made to broaden the bands of optical-fiber communications systems by utilizing a technique such as a wavelength division multiplex (WDM) technique. In the research and development fields related to such optical-fiber communication systems, there have been more and more needs for white light sources including ASE light sources as well as spectrum slice light sources, CDM light sources and the like, all of which are used to evaluate and measure optical parts. In particular, there have been demands for a decrease in the cost of such light sources, a broadening in their bands, and the like.
Specific examples of white light sources including ASE light sources are the configurations shown in FIGS. 1A and 1B. FIG. 1A shows a light source using a single amplified wavelength band, and FIG. 1B shows a broadband light source using two amplified wavelength bands. Referring to FIG. 1A, this light source comprises an active fiber 3 outputting an amplified light output therefrom (hereinafter it denotes an amplified light), a terminator 5 connected to one end of the active fiber 3 and a pump light source 1 and an isolator 4 connected to the other end of the active fiber 3 via a multiplexer 2. The multiplexer 2 couples a pump light emitted from the pump light source 1 to the active fiber 3. Further, the terminator 5 is provided so as to prevent the active fiber 3 from performing unstable operations such as laser oscillation. The isolator 4 is also provided so as to prevent the active fiber 3 from performing unstable operations such as laser oscillation. Further, if reflection of an amplified light from the pump light source is negligible, the isolator 4 and the terminator 5 may be omitted. Conventionally, an erbium (Er)-doped fiber is used as the active fiber 3 outputting a white light, and an amplified light output from the Er-doped fiber is used as a white light.
The operation of this light source will be described in brief taking the Er-doped fiber 3 as an active fiber for instance. The Er-doped fiber is pumped with a pump light from the pump light source 1. In the Er-doped fiber, a pump light generates a local light, which is then amplified while transmitting through the Er-doped fiber in the direction of a fiber axis. The amplified light is emitted to both the multiplexer and terminator sides of the Er-doped fiber (these directions will be referred to as the “forward” and “backward” directions of the figure), and is thus generated in both the forward and backward directions of the Er-doped fiber. Thus, the light source in FIG. 1A uses the single active fiber to obtain an amplified light with a single amplified wavelength band (for example, a C or L band). Further, of the light generated at both sides of the fiber, only the forward output amplified light is used as an output of the light source.
Next, the light source shown in FIG. 1B has a configuration comprising two light sources, each of which is for a single amplified wavelength band as shown in FIG. 1A, and which are connected together in parallel. That is, the light source shown in FIG. 1B comprises a first amplified light generating section 10a having a terminator 5a connected to one end of an active fiber 3a and a pump light source 1a connected to the other end of active fiber 3a via a multiplexer 2a, to output an amplified light output from the active fiber 3a, and a second amplified light generating section 10b having a terminator 5b connected to one end of an active fiber 3b and a pump light source 1b connected to the other end of active fiber 3a via a multiplexer 2b, to output an amplified light output from the active fiber 3b. Furthermore, the amplified light generating sections 10a and 10b are connected together in parallel via a multiplexer 6 to which the isolator 4 is connected at the output side of the multiplexer. An amplified light multiplexed by the multiplexer 6 is output via the isolator 4. The light source shown in FIG. 1B uses the two active fibers 3a and 3b to obtain white light having two amplified wavelength bands (for example, the C and L bands) (see M. Yamada et al., Broadband and gain-flattened amplifier composed of a 1.55 μm-band and a 1.58 μm-band Er3+-doped fibre amplifier in a parallel configuration, Electronic Letters, Vol. 33, No. 8, pp. 710 to 711 (Apr. 10, 1997)). Also in the thus constructed light source, conventionally, erbium (Er)-doped fibers are used as the active fibers 3a and 3b, to output white light, and an amplified light output from each Er-doped fiber is used as white light. Further, this light source also generates amplified light in both the forward and backward directions of each amplified light generating section, but of the light generated at both sides thereof, only the forward output amplified light is used as an output of the light source.
As described above, since the conventional method uses only a rare earth-doped fiber such as an Er-doped fiber as an active fiber, the spectrum of the light source is limited to the gain bandwidth of the rare earth-doped fiber, thereby making it difficult to obtain a broadband light source.
Further, although an amplified light from the active fiber is emitted from both ends of the fiber, the conventional method uses only the light from one end and thus does not efficiently generate a broadband light. Further, if the light source is constructed so that two fibers are connected together in parallel, a certain amount of light components generated outside the wavelength separation characteristic of the multiplexer are discarded, resulting in inefficient light generation. For example, in the above case with an Er-doped fiber, for an Er-doped fiber for the C band, light components having longer wavelengths than this band are discarded, and for an Er-doped fiber for the L band, light components having shorter wavelengths than this band are discarded.